Holographic Projector

ABSTRACT

There is provided a holographic projector comprising a processing engine, spatial light modulator (403B), light source (401B) and light-receiving surface (405B). The processing engine outputs a computer-generated diffractive pattern defining a propagation distance to an image plane. The spatial light modulator displays the computer-generated diffractive pattern. The light source illuminates the spatial light modulator at an angle of incidence (theta) greater than zero. The light-receiving surface receives spatially-modulated light from the spatial light modulator. The light-receiving surface is substantially parallel to the spatial light modulator (alpha-theta). The light-receiving surface is separated from the spatial light modulator by the propagation distance defined by the computer-generated diffractive pattern

FIELD

The present disclosure relates to a projector. More specifically, the present disclosure relates to a holographic projector and holographic projection system. Some embodiments relate to a head-up display and a head-mounted display. Some embodiments relate a method of reducing the size of the image spots in a holographic replay field and some embodiments relates to a method of increasing the resolution in a holographic replay field.

BACKGROUND AND INTRODUCTION

Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object.

Computer-generated holography may numerically simulate the interference process. A computer-generated hologram, “CGH”, may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel or Fourier holograms. A Fourier hologram may be considered a Fourier domain representation of the object or a frequency domain representation of the object. A CGH may also be calculated by coherent ray tracing or a point cloud technique, for example.

A CGH may be encoded on a spatial light modulator, “SLM”, arranged to modulate the amplitude and/or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example.

The SLM may comprise a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The SLM may be reflective meaning that modulated light is output from the SLM in reflection. The SLM may equally be transmissive meaning that modulated light is output from the SLM is transmission.

A holographic projector for imaging may be provided using the described technology. Such projectors have found application in head-up displays, “HUD”, and head-mounted displays, “HMD”, including near-eye devices, for example.

There is disclosed herein an improved holographic projection system.

SUMMARY

Aspects of the present disclosure are defined in the appended independent claims.

There is provided a holographic projector comprising a processing engine, spatial light modulator, light source and light-receiving surface. The processing engine outputs a computer-generated diffractive pattern defining (or incorporating) a propagation distance to an image plane. The spatial light modulator displays the computer-generated diffractive pattern. The light source illuminates the spatial light modulator at an angle of incidence greater than zero. The light-receiving surface receives spatially-modulated light from the spatial light modulator. The light-receiving surface is substantially parallel to the spatial light modulator. The light-receiving surface is separated from the spatial light modulator by the propagation distance defined by the computer-generated diffractive pattern. Broad reference is made herein to a light-receiving surface because the holographic reconstruction may be formed on any surface.

The spatial light modulator is illuminated with collimated light. The person skilled in the art of optics will be familiar with the concept of “normal incidence”. However, the present disclosure relates to so-called “off-axis illumination”. Specifically, the present disclosure relates to off-axis illumination of a spatial light modulator displaying a diffractive pattern including a hologram. The term “off-axis illumination” is used herein to refer to cases in which the angle of incidence of light on the spatial light modulator is non-zero or greater than zero. More specifically, the azimuth angle between a ray of the incident light and the normal to the plane of the spatial light modulator at the point of incidence is non-zero or greater than zero. It may therefore be said that the present disclosure relates to “non-normal incidence” of the spatial light modulator.

The light-receiving surface receives spatially-modulated light from the spatial light modulator. Light which is not diffracted by the spatial light modulator (e.g. the so-called DC spot in the zero-order replay field) follows an output path from the spatial light modulator which is defined herein as the “output optical axis” of the spatial light modulator. The output optical axis is a straight-line between the spatial light modulator and light-receiving surface defining a general propagation direction of light from the spatial light modulator.

In projection, it is conventional to orientate the light-receiving surface such that the normal to the light-receiving surface is parallel to the output optical axis. This conventionally forms the best image—particularly if the image is formed of image spots or pixels. However, the inventors have actually found that with holographic projection this geometry gives rise to sub-optimal holographic reconstruction because there is an adverse effect on the size of the image spots formed by the holographic process—particularly, the image spots at the edge of the holographic replay field. The inventors herein disclose that, in holographic projection using off-axis illumination of the spatial light modulator, the size of the holographic image spots in the replay field can be reduced by tending towards parallelism between the spatial light modulator and light-receiving surface. Smaller image spots (c.f. image pixels) are advantageous in a display device.

The computer-generated diffractive pattern may be either (i) a Fourier hologram combined with a software lens function or (ii) a Fresnel hologram. In both cases, a physical lens (or any component having a lensing effect) is not included in the propagation path from the spatial light modulator to the light-receiving surface. It may be said that the propagation of light from the spatial light modulator to the light-receiving surface is a propagation only through free space. In both cases, it may be said that the distance from the spatial light modulator to the plane containing the light-receiving surface is entirely determined by the light modulation pattern displayed. More specifically, the perpendicular distance or shortest straight-line distance between the spatial light modulator and light-receiving surface is entirely determined by the diffractive pattern. It may be considered that a lens component (or component providing a lensing effect on received light) is embedded or contained in the computer-generated diffractive pattern and that lens component solely determines the distance from the spatial light modulator to the light-receiving surface.

Light modulation data representing the computer-generated diffractive pattern is provided to the spatial light modulator. The light modulation data comprises an array of data values such as a 2D array of data values. The spatial light modulator may comprise a plurality of pixels and each light modulation data value may be assigned to a corresponding pixel. In other words, each pixel of the spatial light modulator may be operated at a light modulation level corresponding to a respective light modulation data value of the array of light modulation data values. The data values may be phase-delay values or amplitude-attenuation levels or both.

In the Fourier case, the necessary lensing function is provided entirely in software using lens data added to Fourier hologram data. The so-called propagation distance from the spatial light modulator to the light-receiving surface is determined—more specifically, solely determined or entirely determined—by the focusing power of the software lens emulated by the lens data. The propagation distance is equal to the focal length of the software lens. The propagation distance is equal to the Fourier path length which is, in turn, equal to the focal length of the software lens. The method may further comprise the lens performing a frequency-space transform of the Fourier transform hologram. In this case, the distance from the spatial light modulator to the light-receiving surface is equal to the focal length of the lens.

In the Fresnel case, the propagation distance is a term in the Fresnel transform used to calculate the hologram. This term determines the distance from the hologram plane to the image plane. That is, the distance from the spatial light modulator to the focal plane where the light-receiving surface should be positioned. It may therefore be said that the distance from the spatial light modulator to the light-receiving surface is equal to the propagation distance, z, encoded in the Fresnel transform.

The angle may be less than 60 degrees, such as less than 45 degrees or less than 30 degrees. In embodiments described by way of example only, the angle is equal to or less than 20 degrees. In these cases, a more compact system is provided. In practice, the angle may be optimised as part of a larger system design.

The spatial light modulator may be a phase modulator and the light modulation data may comprise an array of phase-delay data values. In the Fourier case, phase-delay data corresponding to a lens may be readily calculated and combined with the hologram data of the hologram by wrapped addition which is not computationally demanding. Accordingly, a phase modulating scheme may be preferred.

The light-receiving surface may be diffuse. For example, the light-receiving surface may be a diffuser. The light-receiving surface may be moving such as rotating or oscillating. Accordingly, no keystone effect or image stretching is observed in the replay field.

The spatial light modulator may be a liquid crystal on silicon spatial light modulator and the spatial light modulator is illuminated with coherent light. The light source may be a laser such as a laser diode.

The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, about the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The term “replay plane” is used herein to refer to the plane in space where the holographic reconstruction is fully formed. The term “replay field” is used herein to refer to the sub-area of the replay plane which can receive spatially-modulated light from the spatial light modulator. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light forming the holographic reconstruction. In embodiments, the “image” may comprise discrete spots which may be referred to as “image pixels”.

The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a respect plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to “display” a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to “display” a hologram.

It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography.

The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated.

Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2π) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of π/2 will change the phase of received light by π/2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator.

The present disclosure refers to “lens data corresponding to a lens having a focal length”. This wording is used to reflect that the lens data emulates or provides the functionality (i.e. focusing power) of a lens such as a physical lens in the optical path. The lens data is also referred to as a software lens. A software lens is used in preference to any physical lens.

Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described by way of example only with reference to the following figures:

FIG. 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen;

FIG. 2A illustrates a first iteration of an example Gerchberg-Saxton type algorithm;

FIG. 2B illustrates the second and subsequent iterations of the example Gerchberg-Saxton type algorithm;

FIG. 2C illustrates alternative second and subsequent iterations of the example Gerchberg-Saxton type algorithm;

FIG. 3 is a schematic of a reflective LCOS SLM;

FIG. 4A shows the basic optical set-up in accordance with the present disclosure;

FIG. 4B is a schematic corresponding to FIG. 4A;

FIG. 5 is a schematic of a replay field comprising a plurality of field points;

FIGS. 6A, 6B and 6C shows the image spots at the replay field in accordance with embodiments;

FIGS. 7A, 7B and 7C shows the image spots at the replay field in an alternative example;

FIGS. 8A, 8B and 8C shows the image spots at the replay field in a further alternative example; and

FIGS. 9A, 9B and 9C shows the image spots at the replay field in a yet further alternative example.

The same reference numbers will be used throughout the drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration.

A structure described as being formed at an upper portion/lower portion of another structure or on/under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between.

In describing a time relationship—for example, when the temporal order of events is described as “after”, “subsequent”, “next”, “before” or suchlike—the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as “just”, “immediate” or “direct” is used.

Although the terms “first”, “second”, etc. may be used herein to describe various elements, these elements are not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in co-dependent relationship.

Optical Configuration

FIG. 1 shows an example in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, “LCOS”, device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser.

A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wave-front of light to be incident on the SLM. In FIG. 1, the direction of the wave-front is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wave-front is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the example shown in FIG. 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a light-modulating layer to form an exit wave-front 112. The exit wave-front 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125.

Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field.

The position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In FIG. 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform.

Hologram Calculation

In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms.

A Fourier transform hologram may be calculated using an algorithm such as the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxton algorithm may be used to calculate a hologram in the Fourier domain (i.e. a Fourier transform hologram) from amplitude-only information in the spatial domain (such as a photograph). The phase information related to the object is effectively “retrieved” from the amplitude-only information in the spatial domain. In some embodiments, a computer-generated hologram is calculated from amplitude-only information using the Gerchberg-Saxton algorithm or a variation thereof.

The Gerchberg Saxton algorithm considers the situation when intensity cross-sections of a light beam, I_(A)(x, y) and I_(B)(x, y), in the planes A and B respectively, are known and I_(A)(x, y) and I_(B)(x, y) are related by a single Fourier transform. With the given intensity cross-sections, an approximation to the phase distribution in the planes A and B, ψ_(A)(x, y) and ψ_(B)(x, y) respectively, is found. The Gerchberg-Saxton algorithm finds solutions to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), representative of I_(A)(x, y) and I_(B)(x, y), between the spatial domain and the Fourier (spectral or frequency) domain. The corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing an input image. The hologram may be an amplitude-only hologram, a phase-only hologram or a fully complex hologram.

In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm such as described in British patent 2,498,170 or 2,501,112 which are hereby incorporated in their entirety by reference. However, embodiments disclosed herein describe calculating a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information ψ[u, v] of the Fourier transform of the data set which gives rise to a known amplitude information T[x, y], wherein the amplitude information T[x, y] is representative of a target image (e.g. a photograph). Since the magnitude and phase are intrinsically combined in the Fourier transform, the transformed magnitude and phase contain useful information about the accuracy of the calculated data set. Thus, the algorithm may be used iteratively with feedback on both the amplitude and the phase information. However, in these embodiments, only the phase information ψ[u, v] is used as the hologram to form a holographic representative of the target image at an image plane. The hologram is a data set (e.g. 2D array) of phase values.

In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a fully-complex hologram. A fully-complex hologram is a hologram having a magnitude component and a phase component. The hologram is a data set (e.g. 2D array) comprising an array of complex data values wherein each complex data value comprises a magnitude component and a phase component.

In some embodiments, the algorithm processes complex data and the Fourier transforms are complex Fourier transforms. Complex data may be considered as comprising (i) a real component and an imaginary component or (ii) a magnitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm.

FIG. 2A illustrates the first iteration of an algorithm in accordance with some embodiments for calculating a phase-only hologram. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, wherein each pixel or data value is a magnitude, or amplitude, value. That is, each pixel or data value of the input image 210 does not have a phase component. The input image 210 may therefore be considered a magnitude-only or amplitude-only or intensity-only distribution. An example of such an input image 210 is a photograph or one frame of video comprising a temporal sequence of frames. The first iteration of the algorithm starts with a data forming step 202A comprising assigning a random phase value to each pixel of the input image, using a random phase distribution (or random phase seed) 230, to form a starting complex data set wherein each data element of the set comprising magnitude and phase. It may be said that the starting complex data set is representative of the input image in the spatial domain.

First processing block 250 receives the starting complex data set and performs a complex Fourier transform to form a Fourier transformed complex data set. Second processing block 253 receives the Fourier transformed complex data set and outputs a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, second processing block 253 quantises each phase value and sets each amplitude value to unity in order to form hologram 280A. Each phase value is quantised in accordance with the phase-levels which may be represented on the pixels of the spatial light modulator which will be used to “display” the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantised into one phase level of the 256 possible phase levels. Hologram 280A is a phase-only Fourier hologram which is representative of an input image. In other embodiments, the hologram 280A is a fully complex hologram comprising an array of complex data values (each including an amplitude component and a phase component) derived from the received Fourier transformed complex data set. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form hologram 280A. The step of constraining may include setting each complex data value to the nearest allowable complex modulation level in the complex plane. It may be said that hologram 280A is representative of the input image in the spectral or Fourier or frequency domain. In some embodiments, the algorithm stops at this point.

However, in other embodiments, the algorithm continues as represented by the dotted arrow in FIG. 2A. In other words, the steps which follow the dotted arrow in FIG. 2A are optional (i.e. not essential to all embodiments).

Third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex data set. It may be said that the inverse Fourier transformed complex data set is representative of the input image in the spatial domain.

Fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts the distribution of magnitude values 211A and the distribution of phase values 213A. Optionally, the fourth processing block 259 assesses the distribution of magnitude values 211A. Specifically, the fourth processing block 259 may compare the distribution of magnitude values 211A of the inverse Fourier transformed complex data set with the input image 510 which is itself, of course, a distribution of magnitude values. If the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is acceptable. That is, if the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is a sufficiently-accurate representative of the input image 210. In some embodiments, the distribution of phase values 213A of the inverse Fourier transformed complex data set is ignored for the purpose of the comparison. It will be appreciated that any number of different methods for comparing the distribution of magnitude values 211A and the input image 210 may be employed and the present disclosure is not limited to any particular method. In some embodiments, a mean square difference is calculated and if the mean square difference is less than a threshold value, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the hologram 280A is not acceptable, a further iteration of the algorithm may performed. However, this comparison step is not essential and in other embodiments, the number of iterations of the algorithm performed is predetermined or preset or user-defined.

FIG. 2B represents a second iteration of the algorithm and any further iterations of the algorithm. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of the distribution of magnitude values of the input image 210. In the first iteration, the data forming step 202A formed the first complex data set by combining distribution of magnitude values of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data forming step 202B comprises forming a complex data set by combining (i) the distribution of phase values 213A from the previous iteration of the algorithm with (ii) the distribution of magnitude values of the input image 210.

The complex data set formed by the data forming step 202B of FIG. 2B is then processed in the same way described with reference to FIG. 2A to form second iteration hologram 280B. The explanation of the process is not therefore repeated here. The algorithm may stop when the second iteration hologram 280B has been calculated. However, any number of further iterations of the algorithm may be performed. It will be understood that the third processing block 256 is only required if the fourth processing block 259 is required or a further iteration is required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is usually reached at which no measurable improvement is observed or the positive benefit of performing a further iteration is out-weighted by the negative effect of additional processing time. Hence, the algorithm is described as iterative and convergent.

FIG. 2C represents an alternative embodiment of the second and subsequent iterations. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of an alternative distribution of magnitude values. In this alternative embodiment, the alternative distribution of magnitude values is derived from the distribution of magnitude values 211 of the previous iteration. Specifically, processing block 258 subtracts the distribution of magnitude values of the input image 210 from the distribution of magnitude values 211 of the previous iteration, scales that difference by a gain factor α and subtracts the scaled difference from the input image 210. This is expressed mathematically by the following equations, wherein the subscript text and numbers indicate the iteration number:

R _(n+1)[x, y]=F′{exp(iψ _(n)[u, v])}

ψ_(n)[u, v]=∠F{η·exp(i∠R _(n)[x, y])}

η=T[x, y]−α(|R _(n)[x, y]|−T[x, y])

where:

F′ is the inverse Fourier transform;

F is the forward Fourier transform;

R[x, y] is the complex data set output by the third processing block 256;

T[x, y] is the input or target image;

∠ is the phase component;

ψ is the phase-only hologram 280B;

η is the new distribution of magnitude values 211B; and

α is the gain factor.

The gain factor α may be fixed or variable. In some embodiments, the gain factor α is determined based on the size and rate of the incoming target image data. In some embodiments, the gain factor α is dependent on the iteration number. In some embodiments, the gain factor α is solely function of the iteration number.

The embodiment of FIG. 2C is the same as that of FIG. 2A and FIG. 2B in all other respects. It may be said that the phase-only hologram ψ(u, v) comprises a phase distribution in the frequency or Fourier domain.

In accordance with the present disclosure, the hologram includes data representative of a lens as well as data representing the object. The physical Fourier transform lens 120 shown in FIG. 1 is not present. It is known in the field of computer-generated hologram how to calculate holographic data representative of a lens. The holographic data representative of a lens may be referred to as a software lens. For example, a phase-only holographic lens may be formed by calculating the phase delay caused by each point of the lens owing to its refractive index and spatially-variant optical path length. For example, the optical path length at the centre of a convex lens is greater than the optical path length at the edges of the lens. An amplitude-only holographic lens may be formed by a Fresnel zone plate. It is also known in the art of computer-generated hologram how to combine holographic data representative of a lens with holographic data representative of the object so that a Fourier transform can be performed without the need for a physical Fourier lens. In some embodiments, lensing data is combined with the holographic data by simple addition such as simple vector addition. In further embodiments, the hologram may include grating data—that is, data arranged to perform the function of a grating such as beam steering. Again, it is known in the field of computer-generated holography how to calculate such holographic data and combine it with holographic data representative of the object. For example, a phase-only holographic grating may be formed by modelling the phase delay caused by each point on the surface of a blazed grating. An amplitude-only holographic grating may be simply superimposed on an amplitude-only hologram representative of an object to provide angular steering of an amplitude-only hologram.

In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms.

Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography.

Light Modulation

A spatial light modulator may be used to display the computer-generated hologram. If the hologram is a phase-only hologram, a spatial light modulator which modulates phase is required. If the hologram is a fully-complex hologram, a spatial light modulator which modulates phase and amplitude may be used or a first spatial light modulator which modulates phase and a second spatial light modulator which modulates amplitude may be used.

In some embodiments, the light-modulating elements (i.e. the pixels) of the spatial light modulator are cells containing liquid crystal. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically-active component is the liquid crystal. Each liquid crystal cell is configured to selectively-provide a plurality of light modulation levels. That is, each liquid crystal cell is configured at any one time to operate at one light modulation level selected from a plurality of possible light modulation levels. Each liquid crystal cell is dynamically-reconfigurable to a different light modulation level from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator but the present disclosure is not restricted to this type of spatial light modulator.

A LCOS device provides a dense array of light modulating elements, or pixels, within a small aperture (e.g. a few centimeters in width). The pixels are typically approximately 10 microns or less which results in a diffraction angle of a few degrees meaning that the optical system can be compact. It is easier to adequately illuminate the small aperture of a LCOS SLM than it is the larger aperture of other liquid crystal devices. An LCOS device is typically reflective which means that the circuitry which drives the pixels of a LCOS SLM can be buried under the reflective surface. The results in a higher aperture ratio. In other words, the pixels are closely packed meaning there is very little dead space between the pixels. This is advantageous because it reduces the optical noise in the replay field. A LCOS SLM uses a silicon backplane which has the advantage that the pixels are optically flat. This is particularly important for a phase modulating device.

A suitable LCOS SLM is described below, by way of example only, with reference to FIG. 3. An LCOS device is formed using a single crystal silicon substrate 302. It has a 2D array of square planar aluminium electrodes 301, spaced apart by a gap 301 a, arranged on the upper surface of the substrate. Each of the electrodes 301 can be addressed via circuitry 302 a buried in the substrate 302. Each of the electrodes forms a respective planar mirror. An alignment layer 303 is disposed on the array of electrodes, and a liquid crystal layer 304 is disposed on the alignment layer 303. A second alignment layer 305 is disposed on the planar transparent layer 306, e.g. of glass. A single transparent electrode 307 e.g. of ITO is disposed between the transparent layer 306 and the second alignment layer 305.

Each of the square electrodes 301 defines, together with the overlying region of the transparent electrode 307 and the intervening liquid crystal material, a controllable phase-modulating element 308, often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel which is optically active, taking into account the space between pixels 301 a. By control of the voltage applied to each electrode 301 with respect to the transparent electrode 307, the properties of the liquid crystal material of the respective phase modulating element may be varied, thereby to provide a variable delay to light incident thereon. The effect is to provide phase-only modulation to the wavefront, i.e. no amplitude effect occurs.

The described LCOS SLM outputs spatially modulated light in reflection. Reflective LCOS SLMs have the advantage that the signal lines, gate lines and transistors are below the mirrored surface, which results in high fill factors (typically greater than 90%) and high resolutions. Another advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer can be half the thickness than would be necessary if a transmissive device were used. This greatly improves the switching speed of the liquid crystal (a key advantage for the projection of moving video images). However, the teachings of the present disclosure may equally be implemented using a transmissive LCOS SLM.

Relative Tilt of Spatial Light Modulator and Light-Receiving Surface

FIG. 4A shows three light channels. Each light channel comprises a light source, spatial light modulator and light-receiving surface. Each light channel provides a holographic reconstruction in one colour. Accordingly, a composite colour holographic reconstruction may be provided by using a plurality of single colour channels such as red, green and blue channels and overlapping the single colour replay fields at the replay plane. The hologram of each channel is tailored to the colour content of that channel. FIG. 4A shows three light channels by way of example only. The three channels are substantially parallel and may share a common spatial light modulator—for example, a subset of pixels of the common spatial light modulator may be allocated to each respective colour channel—or each channel may have its own spatial light modulator. The three corresponding replay fields may be coincident at the replay plane. The teachings of the present disclosure are equally applicable to a holographic projector comprising one light channel or any number of light channels. For simplicity, reference is made in the following to the components of just one of the light channels.

FIG. 4A shows a light source 401A illuminating a corresponding liquid crystal on silicon spatial light modulator 403A. The light is incident on the spatial light modulator 403A at an angle greater than zero to the normal of the spatial light modulator 403A. The spatial light modulator 403A is planar and reflective so the spatially-modulated light is output at the same angle to the normal of the spatial light modulator 403A. The spatially modulated light is received by the light-receiving surface 405A.

The same is shown schematically in FIG. 4B. A light source 401B illuminates a spatial light modulator 403B at an angle of θ to the normal of the spatial light modulator 403B. A light-receiving surface 405B receives the spatially-modulated light from the spatial light modulator 403B at an angle of α to the normal of the light-receiving surface 405B. In embodiments, the spatial light modulator 403B and light-receiving surface 405B are parallel. That is, θ is equal to α. In examples shown, θ is 20 degrees but θ may have any non-zero value.

Each pixel of the spatial light modulator 403A/403B displays a respective light modulation level of light modulation data. The light modulation data comprises hologram data corresponding to an image for projection. The light modulation data also comprises lens data corresponding to a lens having an optical power. The lens has a focal length. The holographic reconstruction is formed at the plane of the light-receiving surface 405A/405B owing to the focusing power of the lens data. As explained above, the lens data may be termed a “software lens” and is a mathematical function representative of a physical lens. The software lens provides the same functionality—namely, focusing power—as a physical optical lens of the same dioptric power. The software lens may be an array of phase-delay values corresponding to the shape of the corresponding optical component. A holographic reconstruction of the image is formed on the light-receiving surface 405A/405B. The lens may perform a mathematical transform—such as a Fourier transform—of the hologram. It will be understood that a Fourier transform is a frequency-space transform. In embodiments using a Fourier transform hologram, it may be said that the hologram is a frequency domain representation of the image for projection, the holographic projection is a spatial domain representation of the image and the software lens performs a frequency-space transform of the hologram.

Again, the present disclosure relates to a specific case in which θ is non-zero (in other words, greater than zero) and the inventors have observed that, in this specific case, the size of the image spots in the holographic reconstruction of a hologram displayed on the spatial light modulator can be reduced by tending towards parallelism between the spatial light modulator and light-receiving surface. Ray tracing software has been used to verify this finding. The inventors found that, in the off-axis scheme, the size of the image spots is diffraction-limited when the spatial light modulator and light-receiving surface are parallel. A sample of results are provided herein by way of example but the technical effect achieved has been observed over the full scope of the claims. In particular, the technical effect observed has been found at all possible angles of incidence on the spatial light modulator greater than zero—that is, the off-axis angle of incidence on the spatial light modulator may be any angle greater than zero and less than 90 degrees.

FIG. 5 is a schematic of a holographic replay field 501 and is provided for the purpose of better understanding FIGS. 6 to 9. The replay field 501 in FIG. 5 shows nine so-called field points, labelled FP1 to FP9, which are points in the replay field. Ray tracing has been used to determine the size and shape of the image spot at each of field points FP1 to FP9 for a range of tilt angles.

FIGS. 6 to 9 show nine image spots corresponding to the nine field points, FP1 to FP9, of FIG. 5. The image spots corresponding to the field points are display in ascending numerical order from left to right. That is, the image spot at FP1 is shown to the extreme left and the image spot at FP9 is shown to the extreme right.

Each of FIGS. 6 to 9 show three sets of image spots in three respective rows. The top set of image spots (FIGS. 6A, 7A, 8A and 9A) are formed using blue light having a wavelength of 450 nm, the middle set of image spots (FIGS. 6B, 7B, 8B and 9B) are formed using green light having a wavelength of 520 nm and the bottom set of image spots (FIGS. 6C, 7C, 8C and 9C) are formed using red light having a wavelength of 650 nm. The solid circle or dot shown for each image spot is the corresponding diffraction limit.

The results shown in FIGS. 6 to 9 are achieved in the cases shown in Table 1. The angles shown in Table 1 are angles in degrees to the normal. The relative angle is the angle of the SLM relative to the light-receiving surface and amounts to the difference between θ and α.

TABLE 1 angles used to achieve the results of FIGS. 6 to 9 and Tables 2 to 5. FIG. θ α Relative angle 6 20 20 0 7 5 10 5 8 20 40 20 9 −20 40 −20

The size of the image spot at each field point has been measured and these measurements are shown below in micrometres. The “relative angle” in Tables 2 to 5 is the angle, in degrees, of the light-receiving surface relative to the spatial light modulator.

For the avoidance of any doubt, all image spots in a holographic replay field are formed at the same time from the same computer-generated diffractive pattern. For example, the nine image spots at the nine field points FP1 to FP9 are formed at the same time. This is in contrast to beam scanning systems in which each image is formed bit by bit.

TABLE 2 Image spot size measurements corresponding to FIG. 6 (diffraction limit is 4.287 μm) FIG. FP Relative angle Spot size 6A 1 0 3.446 6A 2 0 3.509 6A 3 0 5.057 6A 4 0 3.509 6A 5 0 2.002 6A 6 0 5.122 6A 7 0 5.122 6A 8 0 2.072 6A 9 0 2.072 6B 1 0 3.228 6B 2 0 3.321 6B 3 0 5.103 6B 4 0 3.321 6B 5 0 1.583 6B 6 0 5.19 6B 7 0 5.19 6B 8 0 1.814 6B 9 0 1.814 6C 1 0 2.987 6C 2 0 3.15 6C 3 0 5.366 6C 4 0 3.15 6C 5 0 1.886 6C 6 0 5.526 6C 7 0 5.526 6C 8 0 2.221 6C 9 0 2.221

TABLE 3 Image spot size measurements corresponding to FIG. 7 (diffraction limit is 4.141 μm). FIG. FP Relative angle Spot size 7A 1 5 0.657 7A 2 5 0.776 7A 3 5 14.241 7A 4 5 0.776 7A 5 5 13.401 7A 6 5 14.407 7A 7 5 14.07 7A 8 5 13.42 7A 9 5 13.42 7B 1 5 0.462 7B 2 5 0.62 7B 3 5 16.172 7B 4 5 0.62 7B 5 5 15.676 7B 6 5 16.387 7B 7 5 16.387 7B 8 5 15.693 7B 9 5 15.693 7C 1 5 0.442 7C 2 5 0.488 7C 3 5 19.914 7C 4 5 0.488 7C 5 5 19.75 7C 6 5 20.238 7C 7 5 20.238 7C 8 5 19.762 7C 9 5 19.762

TABLE 4 Image spot size measurements corresponding to FIG. 8 (diffraction limit is 4.027 μm). FIG. FP Relative angle Spot size 8A 1 20 3.556 8A 2 20 3.991 8A 3 20 66.671 8A 4 20 3.981 8A 5 20 61.004 8A 6 20 62.36 8A 7 20 62.36 8A 8 20 61.43 8A 9 20 61.43 8B 1 20 3.376 8B 2 20 3.941 8B 3 20 70.89 8B 4 20 3.941 8B 5 20 70.798 8B 6 20 71.813 8B 7 20 71.813 8B 8 20 71.274 8B 9 20 71.274 8C 1 20 3.167 8C 2 20 4.071 8C 3 20 88.182 8C 4 20 4.071 8C 5 20 89.031 8C 6 20 89.62 8C 7 20 89.62 8C 8 20 89.455 8C 9 20 89.455

TABLE 5 Image spot size measurements corresponding to FIG. 9 (diffraction limit is 4.284 μm). FIG. FP Relative angle Spot size 9A 1 −20 3.565 9A 2 −20 3.981 9A 3 −20 55.512 9A 4 −20 3.981 9A 5 −20 61.367 9A 6 −20 56.039 9A 7 −20 56.039 9A 8 −20 62.029 9A 9 −20 62.029 9B 1 −20 3.376 9B 2 −20 3.941 9B 3 −20 64.82 9B 4 −20 3.941 9B 5 −20 70.132 9B 6 −20 65.416 9B 7 −20 65.416 9B 8 −20 70.982 9B 9 −20 70.982 9C 1 −20 3.167 9C 2 −20 4.071 9C 3 −20 82.011 9C 4 −20 4.071 9C 5 −20 86.528 9C 6 −20 82.733 9C 7 −20 82.733 9C 8 −20 87.797 9C 9 −20 87.797

The results in Tables 2 to 5 show that, for off-axis illumination, smaller image spots are formed if the spatial light modulator and light-receiving surface are parallel (i.e. the relative angle is zero). Smaller image spots are preferable because they provide higher resolution in the holographic replay field.

Additional Features

Embodiments refer to an electrically-activated LCOS spatial light modulator by way of example only. The teachings of the present disclosure may equally be implemented on any spatial light modulator capable of displaying a computer-generated hologram in accordance with the present disclosure such as any electrically-activated SLMs, optically-activated SLM, digital micromirror device or microelectromechanical device, for example.

In some embodiments, the light source is a laser such as a laser diode. In some embodiments, the light receiving surface is a diffuser surface or screen such as a diffuser. The holographic projection system of the present disclosure may be used to provide an improved head-up display (HUD) or head-mounted display. In some embodiments, there is provided a vehicle comprising the holographic projection system installed in the vehicle to provide a HUD. The vehicle may be an automotive vehicle such as a car, truck, van, lorry, motorcycle, train, airplane, boat, or ship.

The quality of the holographic reconstruction may be affect by the so-called zero order problem which is a consequence of the diffractive nature of using a pixelated spatial light modulator. Such zero-order light can be regarded as “noise” and includes for example specularly reflected light, and other unwanted light from the SLM.

In the example of Fourier holography, this “noise” is focussed at the focal point of the Fourier lens leading to a bright spot at the centre of the holographic reconstruction. The zero-order light may be simply blocked out however this would mean replacing the bright spot with a dark spot. Some embodiments include an angularly selective filter to remove only the collimated rays of the zero order. Embodiments also include the method of managing the zero-order described in European patent 2,030,072, which is hereby incorporated in its entirety by reference.

In some embodiments, the size (number of pixels in each direction) of the hologram is equal to the size of the spatial light modulator so that the hologram fills the spatial light modulator. That is, the hologram uses all the pixels of the spatial light modulator. In other embodiments, the size of the hologram is less than the size of the spatial light modulator. In some of these other embodiments, part of the hologram (that is, a continuous subset of the pixels of the hologram) is repeated in the unused pixels. This technique may be referred to as “tiling” wherein the surface area of the spatial light modulator is divided up into a number of “tiles”, each of which represents at least a subset of the hologram. Each tile is therefore of a smaller size than the spatial light modulator.

In some embodiments, the technique of “tiling” is implemented to increase image quality. Specifically, some embodiments implement the technique of tiling to minimise the size of the image pixels whilst maximising the amount of signal content going into the holographic reconstruction.

In some embodiments, the holographic pattern written to the spatial light modulator comprises at least one whole tile (that is, the complete hologram) and at least one fraction of a tile (that is, a continuous subset of pixels of the hologram).

The holographic reconstruction is created within the zeroth diffraction order of the overall window defined by the spatial light modulator. It is preferred that the first and subsequent orders are displaced far enough so as not to overlap with the image and so that they may be blocked using a spatial filter.

In embodiments, the holographic reconstruction is colour. In examples disclosed herein, three different colour light sources and three corresponding SLMs are used to provide composite colour. These examples may be referred to as spatially-separated colour, “SSC”. In a variation encompassed by the present disclosure, the different holograms for each colour are displayed on different area of the same SLM and then combining to form the composite colour image. However, the skilled person will understand that at least some of the devices and methods of the present disclosure are equally applicable to other methods of providing composite colour holographic images.

One of these methods is known as Frame Sequential Colour, “FSC”. In an example FSC system, three lasers are used (red, green and blue) and each laser is fired in succession at a single SLM to produce each frame of the video. The colours are cycled (red, green, blue, red, green, blue, etc.) at a fast enough rate such that a human viewer sees a polychromatic image from a combination of the images formed by three lasers. Each hologram is therefore colour specific. For example, in a video at 25 frames per second, the first frame would be produced by firing the red laser for 1/75th of a second, then the green laser would be fired for 1/75th of a second, and finally the blue laser would be fired for 1/75th of a second. The next frame is then produced, starting with the red laser, and so on.

An advantage of FSC method is that the whole SLM is used for each colour. This means that the quality of the three colour images produced will not be compromised because all pixels of the SLM are used for each of the colour images. However, a disadvantage of the FSC method is that the overall image produced will not be as bright as a corresponding image produced by the SSC method by a factor of about 3, because each laser is only used for a third of the time. This drawback could potentially be addressed by overdriving the lasers, or by using more powerful lasers, but this would require more power to be used, would involve higher costs and would make the system less compact.

An advantage of the SSC method is that the image is brighter due to all three lasers being fired at the same time. However, if due to space limitations it is required to use only one SLM, the surface area of the SLM can be divided into three parts, acting in effect as three separate SLMs. The drawback of this is that the quality of each single-colour image is decreased, due to the decrease of SLM surface area available for each monochromatic image. The quality of the polychromatic image is therefore decreased accordingly. The decrease of SLM surface area available means that fewer pixels on the SLM can be used, thus reducing the quality of the image. The quality of the image is reduced because its resolution is reduced. Embodiments utilise the improved SSC technique disclosed in British patent 2,496,108 which is hereby incorporated in its entirety by reference.

Examples describe illuminating the SLM with visible light but the skilled person will understand that the light sources and SLM may equally be used to direct infrared or ultraviolet light, for example, as disclosed herein. For example, the skilled person will be aware of techniques for converting infrared and ultraviolet light into visible light for the purpose of providing the information to a user. For example, the present disclosure extends to using phosphors and/or quantum dot technology for this purpose.

Some embodiments describe 2D holographic reconstructions by way of example only. In other embodiments, the holographic reconstruction is a 3D holographic reconstruction. That is, in some embodiments, each computer-generated hologram forms a 3D holographic reconstruction.

The methods and processes described herein may be embodied on a computer-readable medium. The term “computer-readable medium” includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term “computer-readable medium” shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part.

The term “computer-readable medium” also encompasses cloud-based storage systems. The term “computer-readable medium” includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions).

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents. 

1. A holographic projector comprising: a processing engine arranged to output a computer-generated diffractive pattern defining a propagation distance to an image plane; a planar spatial light modulator arranged to display the computer-generated diffractive pattern; a light source arranged to illuminate the spatial light modulator at an angle of incidence (θ) greater than zero, wherein the spatial light modulator is arranged to output spatially-modulated light on a propagation path having an axis so as to form a holographic reconstruction of the computer-generated diffractive pattern on the image plane; a screen arranged at the image plane to receive spatially-modulated light from the spatial light modulator, wherein the screen is arranged substantially parallel to the spatial light modulator such that the axis of the propagation path of the spatially-modulated light received from the spatial light modulator is at an angle of incidence (α) on the screen substantially equal to the angle of incidence (θ) of light from the light source on the spatial light modulator, and wherein the screen is substantially parallel to the spatial light modulator and separated from the spatial light modulator by the propagation distance along the axis of the propagation path defined by the computer-generated diffractive pattern, wherein the computer-generated diffractive pattern is one selected from the group comprising: the sum of a Fourier hologram and lens function corresponding to a lens having a focal length equal to the propagation distance; and a Fresnel hologram.
 2. A holographic projector as claimed in claim 1, wherein the angle of incidence (θ) of light on the spatial light modulator is less than 60 degrees.
 3. A holographic projector as claimed in claim 1, wherein the spatial light modulator is a phase modulator and the computer-generated diffractive pattern is a distribution of phase-delay values.
 4. A holographic projector as claimed in claim 1 wherein the processing engine is arranged to calculate the computer-generated diffractive pattern.
 5. A holographic projector as claimed in claim 1 wherein the screen is a diffuser.
 6. A head-up display comprising a holographic projector as claimed in claim
 1. 7. A method of holographic projection, the method comprising: providing a computer-generated diffractive pattern defining a propagation distance to an image plane, wherein the computer-generated diffractive pattern is (i) the combination of a Fourier hologram and lens function corresponding to a lens having a focal length equal to the propagation distance or (ii) a Fresnel hologram; displaying the computer-generated diffractive pattern on a planar spatial light modulator; illuminating the spatial light modulator at an angle of incidence (θ) greater than zero; outputting spatially-modulated light on a propagation path having an axis so as to form a holographic reconstruction of the computer-generated diffractive pattern on the image plane; receiving spatially-modulated light from the spatial light modulator on a screen at the image plane, wherein the screen is substantially parallel to the spatial light modulator such that the axis of the propagation path of the spatially-modulated light received from the spatial light modulator is at an angle of incidence (α) on the screen substantially equal to the angle of incidence (θ) of light from the light source on the spatial light modulator, wherein the screen is separated from the spatial light modulator by the propagation distance along the axis of the propagation path defined by the computer-generated diffractive pattern.
 8. A method of holographic projection as claimed in claim 7, the method further comprising calculating the computer-generated diffractive pattern using a Fourier transform.
 9. A holographic projector comprising: 